Soil solution Zn and pH dynamics in non-rhizosphere soil and in the rhizosphere of Thlaspi caerulescens grown in a Zn/Cd-contaminated soil

Temporal changes in soil solution properties and metal speciation were studied in non-rhizosphere soil and in the rhizosphere of the hyperaccumulator Thlaspi caerulescens J. & C. Presl (population from Prayon, Belgium) grown in a Zn- and Cd-contaminated soil. This paper focuses on soil solution Zn and pH dynamics during phytoextraction. The concentration of Zn in both non-rhizosphere and rhizosphere soil solutions decreased from 23 mg/l at the beginning to 2 mg/l at the end of the experiment (84 days after transplanting of seedlings), mainly due to chemical sorption. There was no significant difference in overall Zn concentration between the planted and the unplanted soil solutions (P > 0.05). Soil solution pH decreased initially and then increased slightly in both planted and unplanted soil zones. From 60 to 84 days after transplanting, the pH of the rhizosphere soil solution was higher than that of non-rhizosphere soil solution (P<0.05). Zn uptake by the hyperaccumulator plants was 8.8 mg per pot (each containing 1 kg oven-dry soil) on average. The data indicate that the potential of T. caerulescens to remove Zn from contaminated soil may not be related to acidification of the rhizosphere.     

Evaluation of hydrophobicity in PAH-contaminated soils during phytoremediation

The impact of recalcitrant organic compounds on soil hydrophobicity was evaluated in contaminated soil from a manufactured gas plant site following 12 months of phytoremediation. Significant reduction in soil wetting and water retention was observed in contaminated soil compared to an uncontaminated control. Phytoremediation was effective at reducing total PAHs by 69% with corresponding changes in soil classification from extremely hydrophobic (initial sample) to moderately-strongly hydrophobic (planted) and hydrophilic-very hydrophilic (unplanted) after 12 months. The greatest reduction in soil hydrophobicity was observed in the unplanted, unfertilized treatments that had the lowest removal rate of PAHs. The presence of plants may contribute to hydrophobicity in contaminated soil.     

Phytoremediation of an aged petroleum contaminated soil using endophyte infected and non-infected grasses

Phytoremediation is a promising technique for cleaning petroleum contaminated soils. In this study, the effects of two grass species (Festuca arundinacea Schreb. and Festuca pratensis Huds.), infected (E(+)) and non-infected (E(-)) by endophytic fungi (Neotyphodium coenophialum and Neotyphodium uncinatum, respectively) on the degradation of petroleum hydrocarbons in an aged petroleum contaminated soil was investigated. Plants were grown in the soil for 7 months and unplanted soil considered as control. At the end of the experiment, total and oil-degrading bacteria, dehydrogenase activity, water-soluble phenols, total petroleum hydrocarbons (TPH) and polycyclic aromatic hydrocarbons (PAHs) contents were measured in the soil. The results demonstrated that E(+) plants contained more root and shoot biomass than E(-) plants and created higher levels of water-soluble phenols and dehydrogenase activity in the soil, while there was no significant difference in bacterial counts of planted soils. Planting stimulated total and oil-degrading bacterial numbers, dehydrogenase activity and the soil content of water-soluble phenols. Regardless of endophyte infection, PAH and TPH removal in the rhizosphere of plants were 80-84 and 64-72% respectively, whereas the removals in controls were 56 and 31%, respectively. It was revealed that TPHs in retention time range of n-alkanes with C(10)-C(25) chain lengths and TPH were more degraded in the rhizosphere of E(+) plants compared to E(-) ones. Thus, grasses infected with endophytic fungi could be more efficient for removal of TPH from oil-contaminated soils.     

Accumulation of phenanthrene and pyrene in rhizosphere soil

A study was conducted to determine PAH concentrations in the rhizosphere of plants grown in soil containing phenanthrene or pyrene. The rhizosphere of tall fescue and wheat grown in sterile soil contained 4-5-fold higher pyrene concentrations than unplanted soil. The rhizosphere of several plant species grown in non-sterile soil temporarily contained appreciably more phenanthrene or pyrene than unplanted soil, but those PAHs were degraded with time. The data suggest that plants accumulate such hydrophobic compounds in the rhizosphere after facilitating their transport toward the roots.     

Effect of arbuscular mycorrhizal fungus (Glomus caledonium) on the accumulation and metabolism of atrazine in maize (Zea mays L.) and atrazine dissipation in soil

Effects of an arbuscular mycorrhizal (AM) fungus (Glomus caledonium) on accumulation and metabolism of atrazine in maize grown in soil contaminated with different concentrations of atrazine were investigated in a series of pot experiments. Roots of mycorrhizal plants accumulated more atrazine than non-mycorrhizal roots. In contrast, atrazine accumulation in shoot decreased in mycorrhizal compared with non-mycorrhizal plants. No atrazine derivatives were detected in the soil, either with or without mycorrhizal colonization. However, atrazine metabolites, deethylatrazine (DEA) and deisopropylatrazine (DIA), were detected in plant roots and the AM colonization enhanced the metabolism. After plant harvest atrazine concentrations decreased markedly in the soils compared to the initial concentrations. The decreases were the most in rhizosphere soil and then near-rhizosphere soil and the least in bulk soil. Mycorrhizal treatment enhanced atrazine dissipation in the near-rhizosphere and bulk soils irrespective of atrazine application rates.     

Fate of isoxaben in a containerized plant rhizosphere system

Commercial production of ornamental plants is an important industry in the United States and involves a complex technology that includes the use of herbicides. Isoxaben[N-[3-(1-ethyl-1-methylpropyl)-5-isoxazolyl]-2,6-dimethoxybenzamide] is a pre-emergence herbicide used for controlling weeds in many areas including containerized ornamental plants. Degradation was studied in potting mix (80% bark, 20% sand) with three different regimes (sterile, bulk and rhizosphere). The rhizosphere regime contained Switch Grass (Panicum virgatum), and plants were allowed to grow for 14 days before adding isoxaben (10 microg/g potting mix). Isoxaben was degraded to 0.5 microg/g in 60 days giving a half-life of 7 days. Two degradation products were detected: 3-nitrophthalic acid in the rhizosphere and bulk regimes and 4-methoxyphenol in the sterile regime. Microbial population shifts were determined by fatty acid methyl ester profile analysis and were influenced by the introduction of a plant (rhizosphere regime) and by isoxaben addition.     

Release behavior of triazine residues in stabilised contaminated soils

This paper reports the release behavior of two triazines (atrazine and simazine) in stabilised soils from a pesticide-contaminated site in South Australia. The soils were contaminated with a range of pesticides, especially with triazine herbicides. With multiple extractions of each soil sample with deionised water (eight in total), 15% of atrazine and 4% of simazine residues were recovered, resulting in very high concentrations of the two herbicides in leachate. The presence of small fractions of surfactants was found to further enhance the release of the residues. Methanol content up to 10% did not substantially influence the concentration of simazine and atrazine released. The study demonstrated that while the stabilisation of contaminated soil with particulate activated carbon (5%) and cement mix (15%) was effective in locking the residues of some pesticides, it failed to immobilise triazine herbicides residues completely. Given the higher water solubility of these herbicides than other compounds more effective strategies to immobilise their residues is needed.     

Assessment of contaminant lability during phytoremediation of polycyclic aromatic hydrocarbon impacted soil

Polycyclic aromatic hydrocarbons (PAHs) are recalcitrant compounds, some of which are known carcinogens, often found in high residual soil concentrations at industrial sites. Recent research has confirmed that phytoremediation holds promise as a low-cost treatment method for PAH contaminated soil. In this study, the lability of soil bound PAHs in the rhizosphere was estimated using solid phase extraction resin. An extraction time of 14 days was determined to be appropriate for this study. Resin-extractable PAHs, which are assumed to be more bioavailable, decreased during plant treatments. Significant reductions in the labile concentrations of several PAH compounds occurred over 12 months of plant growth. The differences in concentration between the unplanted and the planted soil indicate that the presence of plant roots, in addition to the passage of time, contributes to reduction in the bioavailability of target PAHs.     

Degradation of atrazine by an acclimatized soil fungal isolate

A fungal strain able to use atrazine (2-chloro-4-ethylamino-5-isopropylamino-1,3,5-triazine) as a source of nitrogen was isolated from a corn field soil that has been previously treated with the herbicide. This strain was purified and acclimatized to atrazine at a higher level in the laboratory. A supplemented N was required to trigger the reaction. Atrazine was degraded at a faster rate in inoculated mineral salt medium (MSM) than non-inoculated MSM. Within 20 days, nearly 34% of the atrazine was degraded in inoculated medium while only 2% of the herbicide was degraded in non-inoculated medium. Degradation of atrazine by the isolated fungal strain was also studied in sterile and non-sterile soil to determine the compatibility of the isolated strain with native microorganisms in soil. The degradation of atrazine was found to be more in inoculated sterile soil than in inoculated non-sterile soil. Cell free extract (CFE) of fungal mycelium degraded about 50% of the atrazine in buffer in 96 hours compared to the control. Four atrazine metabolites were isolated and characterized by LCMS. On the basis of morphological parameters the isolate was identified as Penicillium species. Results indicated that the microorganism may be useful for remediation of atrazine-contaminated soil.     

Plant-promoted pyrene degradation in soil

A study was conducted to investigate the capability of nine plant species to promote the degradation of pyrene in soil. The test method allowed for analysis of the entire sample of soil. More pyrene was degraded in the presence of roots of all nine species than in unplanted soil. Within approximately 8 weeks, as much as 74% of the pyrene disappeared from vegetated soil compared to 40% or less from unplanted soil. The data suggest that some of the test species may be especially useful for phytoremediation of soils contaminated with PAHs.     

Dissipation of 2,4-D in soils of the Humid Pampa region, Argentina: a microcosm study

Phenoxy herbicides like 2,4-dichlorophenoxyacetic acid (2,4-D) are widely used in agricultural practices. Although its half life in soil is 7-14d, the herbicide itself and its first metabolite 2,4-dichlorophenol (2,4-DCP) could remain in the soil for longer periods, as a consequence of its intensive use. Microcosms assays were conducted to study the influence of indigenous microflora and plants (alfalfa) on the dissipation of 2,4-D from soils of the Humid Pampa region, Argentina, with previous history of phenoxy herbicides application. Results showed that 2,4-D was rapidly degraded, and the permanence of 2,4-DCP in soil depended on the presence of plants and soil microorganisms. Regarding soil microbial community, the presence of 2,4-D degrading bacteria was detected even in basal conditions in this soil, possibly due to the adaptation of the microflora to the herbicide. There was an increment of two orders of magnitude in herbicide degraders after 15d from 2,4-D addition, both in planted and unplanted microcosms. Total heterotrophic bacteria numbers were about 1x10(8) CFUg(-1) dry soil and no significant differences were found between different treatments. Overall, the information provided by this work indicates that the soil under study has an important intrinsic degradation capacity, given by a microbial community adapted to the presence of phenoxy herbicides.     

Enhanced phytoremediation of arsenic contaminated land

In an attempt to clean up arsenic (As) contaminated soil, the effects of phosphorus (P) fertilizer and rhizosphere microbes on arsenic accumulation by the silverback fern, Pityrogramma calomelanos, were investigated in both greenhouse and field experiments. Field experiments were conducted in Ron Phibun District, an As-contaminated area in Thailand. Soil (136-269 microg As g(-1)) was collected there and used in the greenhouse experiment. Rhizosphere microbes (bacteria and fungi) were isolated from roots of P. calomelanos growing in Ron Phibun District. The results showed that P-fertilizer significantly increased plant biomass and As accumulation of the experimental P. calomelanos. Rhizobacteria increased significantly the biomass and As content of the test plants. Thus, P-fertilizer and rhizosphere bacteria enhanced As-phytoextraction. In contrast, rhizofungi reduced significantly As concentration in plants but increased plant biomass. Therefore, rhizosphere fungi exerted their effects on phytostabilization.     

Phytoremediation of pyrene contaminated soils amended with compost and planted with ryegrass and alfalfa

Ryegrass (Lolium perenne) and alfalfa (Medicago sativa) were planted in pots to remediate pyrene contaminated quartz sand (as a control group), alluvial and red soils amended with and without compost. The pyrene degradation percentages in quartz sand, alluvial soil, and red soil amended with compost (5%, w/w) and planted with ryegrass and alfalfa for 90 d growth were 98-99% and 97-99%, respectively, while those of pyrene in the corresponding treatments amended without compost but planted with ryegrass and alfalfa were 91-96% and 58-89%, respectively. Further, those of pyrene in the respective treatments amended with and without compost but unplanted were 54-77% and 51-63%, respectively. Pyrene contents in both roots and aboveground parts of ryegrass and alfalfa after 90 d growth in quartz sand and the two soils amended with or without compost were trace amounts. Statistical analyses for the parameters of ryegrass planted in red and alluvial soils including the concentrations of total water-soluble volatile low molecular weight organic acids, microbial population, pyrene degradation percentage, and spiked pyrene concentration show significant correlations at 5% and mostly 1% probability levels, by the analysis of variance. It was thus suggested that the interactions among the consortia of plant root exudates, microorganisms, and amended compost in rhizosphere soils could facilitate bioavailability of pyrene and subsequently enhance its dissipation.     

Procedures of trophic chain samples preparation for determination of triazines by HPLC and metals by ICP-AES methods

The aim of this research was monitoring the distribution of atrazine and simazine as well as metals Pb, Cd, Zn, Al, Co, Ni, and V along with trophic chains: soil-vegetables and soil, carrot or grass and meat. Different techniques of herbicides extraction by means of many solvents were examined. Triazines were analysed by means of HPLC, metals by means of ICP-AES. Detection limits: LOD=0.2 microg ml(-1), determination limits: LOQ=0.73 microg ml(-1) for atrazine and LOD=0.3 microg ml(-1), LOQ=1.12 microg ml(-1) for simazine were obtained. The content (microg g(-1)) of simazine in soil was in range: 3.45-8.60, in vegetable roots: 6.62-38.15, in vegetable leaves: 2.45-31.71, in rabbit fat: 0.13-49.90. The content (microg g(-1)) of atrazine in soils was in range: 11.9-13.03, in vegetable roots: 13.61-92.90. In analysed material the particular metals after microwave or dry digestion were determined in range (microg g(-1)): Pb: 6.48-43.18; Cd: 0.11-0.57; Zn: 8.79-51.90; Al: 10.22-24.48; Co: 0.18-3.89; Ni: 0.37-6.36; V: 0.29-1.48.     

Distribution of the herbicide atrazine in a microcosm with riparian forest plants

Pesticides applied on sugarcane reach the subsoil of riparian forests and probably contaminate the river water. This work was conducted to learn about the phytoremediation of atrazine and subsoil contamination using the common riparian forest species of Cecropia hololeuca Miq. and Trema micranta (L.) Blum. These plants were grown in soil microcosms where (14)C-atrazine at 1/10 of the field-recommended dose was applied at the bottom of the microcosm simulating the movement from contaminated ground water to the upper soil layers and into plants. Residues of (14)C-atrazine were detected in all parts of the microcosm including soil, rhizosphere and the roots in different layers of the microcosm, stem and leaves. Atrazine mineralization was higher (10.2%) in the microcosms with plants than the control microcosms without plants (1.2%). The upward movement of this pesticide from deeper to more superficial soil layers occurred in all the microcosms with plants, powered by evapotranspiration process. From the atrazine applied in this study about 45% was taken up by C. hololeuca and 35% by T. micrantha. The highest amount of radioactivity (%) was found in the fine roots and the specific radioactivity (% g(-1)) showed that thick, fine roots and leaves bioaccumulate atrazine. The enhanced mineralization of atrazine as well the phytostabilization effect of the tree biomass will reduce the bioavailability of these residues and consequently decrease the hazardous effects on the environment.     

Influence of microbial and synthetic surfactant on the biodegradation of atrazine

The present study reports the effect of surfactants (rhamnolipids and triton X-100) on biodegradation of atrazine herbicide by strain A6, belonging to the genus Acinetobacter. The strain A6 was able to degrade nearly 80 % of the 250-ppm atrazine after 6 days of growth. The bacterium degraded atrazine by de-alkylation process. Bacterial cell surface hydrophobicity as well as atrazine solubility increased in the presence of surfactant. However, addition of surfactant to the mineral salt media reduced the rate and extent of atrazine degradation by decreasing the bioavailability of herbicide. On the contrary, addition of surfactant to atrazine-contaminated soil increased the rate and extent of biodegradation by increasing the bioavailability of herbicide. As compared to triton X-100, rhamnolipids were more efficient in enhancing microbial degradation of atrazine as a significant amount of atrazine was removed from the soil by rhamnolipids. Surfactants added for the purpose of hastening microbial degradation may have an unintended inhibitory effect on herbicide degradation depending upon contiguous condition, thus highlighting the fact that surfactant must be judiciously used in bioremediation of herbicides.     

Distribution of the herbicide atrazine in a microcosm with riparian forest plants

Pesticides applied on sugarcane reach the subsoil of riparian forests and probably contaminate the river water. This work was conducted to learn about the phytoremediation of atrazine and subsoil contamination using the common riparian forest species of Cecropia hololeuca Miq. and Trema micranta (L.) Blum. These plants were grown in soil microcosms where ¹⁴C-atrazine at 1/10 of the field-recommended dose was applied at the bottom of the microcosm simulating the movement from contaminated ground water to the upper soil layers and into plants. Residues of ¹⁴C-atrazine were detected in all parts of the microcosm including soil, rhizosphere and the roots in different layers of the microcosm, stem and leaves. Atrazine mineralization was higher (10.2%) in the microcosms with plants than the control microcosms without plants (1.2%). The upward movement of this pesticide from deeper to more superficial soil layers occurred in all the microcosms with plants, powered by evapotranspiration process. From the atrazine applied in this study about 45% was taken up by C. hololeuca and 35% by T. micrantha. The highest amount of radioactivity (%) was found in the fine roots and the specific radioactivity (% g^?1) showed that thick, fine roots and leaves bioaccumulate atrazine. The enhanced mineralization of atrazine as well the phytostabilization effect of the tree biomass will reduce the bioavailability of these residues and consequently decrease the hazardous effects on the environment.     

Study on the possibility of hydrogen peroxide pretreatment and plant system to remediate soil pollution

Hydrogen peroxide was widely selected as the chemical oxidant in chemical remediation or as the donor of oxygen in in situ aerobic bioremediation of organic pollutants. In this paper, hydrogen peroxide pretreatment and plant system was done to examine its possibility to remediate the heavy metal contaminated soil or heavy metal-organic combined contaminated soil. Heavy metal contaminated soil was collected from the heavily industrialized area, in Fuyang county, Zhejiang province, China. And heavy metal-organic combined contaminated soil was prepared from the same contaminated soil by spiking 100 microg g(-1) 2,4-dichlorophenol (2,4-DCP). Results showed that H2O2 could improve the dissipation of 2,4-DCP and enhance the availability of Cu and Zn in soil. The greatly increased DOC (dissolved organic carbon) in the oxidation process was probably the main reason for the greatly increased water soluble Cu in higher pH condition. Water soluble Zn, however, easily rebound to soil components with the time being and had no positive relation with dissolved organic carbon. Planting with ryegrass influenced the behavior of pollutants in soil. It was observed that the dissipation of 2,4-DCP could be enhanced by the presence of plant roots and the availability of Cu and Zn in the planted soil was changed due to the mobilization and rebound mechanisms in the rhizosphere. Co-contamination of 2,4-DCP caused the greater availability of Cu and Zn in H2O2 pretreatment. But with the ryegrass planting, it was easier to rebound to the less available phase in the rhizosphere. Both Cu and Zn concentration in shoots increased with the H2O2 treatment. Therefore our results suggested that H2O2 pretreatment was probably a promising way for promoting the dissipation of persistent organic pollutants and enhancing the solubility of Cu and Zn in soil. A combination of H2O2 pretreatment and suitable plant might be an efficient alternative for remedying heavy metal or heavy metal-organic contaminated soil.     

Transport of simazine in unsaturated sandy soil and predictions of its leaching under hypothetical field conditions

The potential contamination of groundwater by herbicides is often controlled by processes in the vadose zone, through which herbicides travel before entering groundwater. In the vadose zone, both physical and chemical processes affect the fate and transport of herbicides, therefore it is important to represent these processes by mathematical models to predict contaminant movement. To simulate the movement of simazine, a herbicide commonly used in Chilean vineyards, batch and miscible displacement column experiments were performed on a disturbed sandy soil to quantify the primary parameters and processes of simazine transport. Chloride (Cl(-)) was used as a non-reactive tracer, and simazine as the reactive tracer. The Hydrus-1D model was used to estimate the parameters by inversion from the breakthrough curves of the columns and to evaluate the potential groundwater contamination in a sandy soil from the Casablanca Valley, Chile. The two-site, chemical non-equilibrium model was observed to best represent the experimental results of the miscible displacement experiments in laboratory soil columns. Predictions of transport under hypothetical field conditions using the same soil from the column experiments were made for 40 years by applying herbicide during the first 20 years, and then halting the application and considering different rates of groundwater recharge. For recharge rates smaller than 84 mm year(-1), the predicted concentration of simazine at a depth of 1 m is below the U.S. EPA's maximum contaminant levels (4 microg L(-1)). After eight years of application at a groundwater recharge rate of 180 mm year(-1) (approximately 50% of the annual rainfall), simazine was found to reach the groundwater (located at 1 m depth) at a higher concentration (more than 40 microg L(-1)) than the existing guidelines in the USA and Europe.     

Influence of plants on the chemical extractability and biodegradability of 2,4-dichlorophenol in soil

This study investigated the fate and behaviour of [UL-(14)C] 2,4-dichlorophenol (DCP) in planted (Lolium perenne L.) and unplanted soils over 57 days. Extractability of [UL-(14)C] 2,4-DCP associated activity was measured using calcium chloride (CaCl(2)), acetonitrile-water and dichloromethane (DCM) extractions. Biodegradability of [UL-(14)C] 2,4-DCP associated activity was assessed through measurement of (14)CO(2) production by a degrader inoculum (Burkholderia sp.). Although extractability and mineralisation of [UL-(14)C] 2,4-DCP associated activity decreased significantly in both planted and unplanted soils, plants appeared to enhance the sequestration process. After 57 days, in unplanted soil, 27% of the remaining [UL-(14)C] 2,4-DCP associated activity was mineralised by Burkholderia sp., and 13%, 48%, and 38% of (14)C-activity were extracted by CaCl(2), acetonitrile-water and DCM, respectively. However, after 57 days, in planted soils, only 10% of the [UL-(14)C] 2,4-DCP associated activity was available for mineralisation, whilst extractability was reduced to 2% by CaCl(2), 17% by acetonitrile-water and 11% by DCM. This may be due to the effect of plants on soil moisture conditions, which leads to modification of the soil structure and trapping of the compound. However, the influence of plants on soil biological and chemical properties may also play a role in the ageing process.